Circuit board testers are used for testing a variety of circuit boards or UUT (unit under test which may be something other than a circuit board—the terms will be used interchangeably to refer to all possibilities) or similar devices to assure that the circuit boards operate as intended. In at least one type of circuit board tester, such as Agilent Model No. 3070, Series 3, a separate device, referred to as a fixture, is used to position the circuit board such that a plurality of electrically conductive probes (which are part of, or coupled to, the tester) contact predetermined components or positions of the circuit board. The particular components or positions that are contacted by the test or probes depend on the tests that are desired. When the probes are in contact with the desired locations on the circuit board, electrical signals with predetermined parameters (e.g., predetermined magnitudes or patterns of current, voltage frequency, phase, and the like) are applied by the tester, typically under control of a computer, to certain of the probes. Some or all of the probes are used to measure the performance or response of the circuit board (i.e., to measure electrical parameters at some or all of the probes contacting the circuit board). In this way, it is possible to rapidly perform a number of tests or measurements characterizing the performance of the circuit board while simulating the conditions the circuit board would have, or could have, during actual use. Although it is possible to use these types of tests and testing devices for a variety of possible purposes (such as “spot checking” selected circuit boards at a production facility, testing circuit boards which may be malfunctioning, testing prototype circuit boards as part of a design program, and the like), in at least some applications, circuit board testing is used to provide quality assurance on all or substantially all products of a given type or class which are produced by a company.
Some circuit board testers use a vacuum fixture that uses atmospheric pressure, specifically a vacuum, to drive a platen with a combination of spring-loaded probes and circuit board push down features to press against the circuit board and press it against another platen with a field of spring-loaded test probes. In some circuit board testers, all of the probes contact the circuit board simultaneously. While vacuum systems are effective, they are quite complex and expensive to manufacture.
Some circuit board testers use what is known as a dual-stage test fixture. A dual-stage test fixture typically has spring-loaded test probes that are of two different lengths, known as long-stroke probes and short-stroke probes. As a result, different test cycles can involve different test probes contacting the circuit board. For example, during a powered in-circuit test, both the long- and short-stroke probes contact the circuit board being tested at the same time. On the other hand, during a non-powered test, only the long-stroke probes contact the circuit board. Contacting the circuit board with only the long-stroke probes allows different circuits on the circuit board to be tested without introducing electrical interference by contacting the circuit board with all of the probes.
In some conventional circuit board testers employing dual-stage test fixtures, however, during the non-powered test, both sides of the circuit board may not be fully supported. As a result, some of the short-stroke probes can contact the circuit board during the non-powered test, potentially inadequate contact force and poor signal quality and interference. This interference can cause the test to fail and may cause electrical damage to the electronic components of the circuit board. Failure to fully support the circuit board at all times may cause the circuit board to be subject to deformations typically caused by vacuum pressure differentials that can lead to stress fractures in the solder connections or in the electronic components on the circuit board. By fully supporting the board/UUT, and eliminating pressure stresses thereon, the quality of test is improved, while simultaneously reducing the complexity of the tester.